1. Field of the Invention
The aspects of the present disclosure relate generally to light emitting diode devices and in particular to addressing color shifting of red LEDs in blue shifted yellow and red (BSY-R) light engines.
2. Description of Related Art
Light Emitting Diodes (LED(s)) are widely used in general lighting. An LED is generally understood as a semiconductor device that generates light when electrical energy is applied to the device. LED arrays, in which multiple LEDs are formed into an array and powered as a unit, are gaining popularity in lighting and signaling applications. LED arrays are typically connected to a direct current (DC) power source where the amount of applied current controls the brightness of emitted light.
The level of light that an LED outputs will typically depend upon the amount of electrical current supplied to the LED, also referred to as a diode or chip, and the operating temperature of the LED. Operating temperatures also affect the useful life of an LED. LED light engines that mix phosphor-coated blue dies with red dies tend to encounter problems, in that red dies degrade more quickly because red dies are more sensitive to temperature than blue dies. This degradation will cause the system color to shift away from red over time.
In blue shifted yellow and red light engines, generally referred to as “BSY-R”, the blue and red LED chips are typically mounted to a common platform. The common platform is then connected to the main heat sink. As such, all of the LED chips on the platform experience the same thermal environment. Red LED chips are known to have much stronger “droop” curves than blue dies. “Droop” curves illustrate the light output with temperature over time. Thus, a red LED chip will show a faster rate of decline in light output over time at higher temperatures than, for example, a blue LED chip. Generally, BSY-R includes white LEDs plus red LEDs. It will be understood that the white LEDs are blue LEDs with a phosphor that, together, produce white.
Some applications implement a separate control to manage the electrical current to the red LED chips over time to compensate for thermal degradation. This generally results in an increase in electrical current over time. In some cases, extra red LED chips or optical detectors are required to compensate for the thermal degradation of the red LED chips. However, the increase in required current or additional components adds more cost to the light engine system.
An LED can be mounted on a metal heat sink to dissipate the heat when the diode is run using high current. It is desirable to run LEDs using high current because the brightness of the light emitted from the LEDs is more intense at higher currents. However, as the number of LEDs in an array increase, the operating temperatures tend to increase. The higher operating temperatures can negatively impact LEDs which show a faster rate of decline in light output over time at higher temperatures than other LEDs. The light output of such an LED array will have a tendency to shift away from the color of the degrading LED.
A thermoelectric device or cooler (TEC) can be used with LED devices to provide cooling. A thermoelectric cooler is a device that can force one surface to a particular temperature and has been proposed for use with LED lighting to make the LEDs run cooler. This can generally be referred to as a “thermoelectric effect.” The thermoelectric effect is generally understood to be the direct conversion of temperature differences to electric voltage, and electric voltage to temperature differences. The thermoelectric effect can be used to generate electricity, measure temperature or change the temperature of objects. An example of a thermoelectric device is one that includes two different materials or dissimilar metals that are in the path of an electric circuit and provides direct conversion of voltages to heat differentials. Such thermoelectric devices, as are generally understood, will provide a temperature differential when an electric voltage difference is applied to the two dissimilar metals. The term “thermoelectric effect” is generally understood to encompass three different effects, the Peltier effect, the Seebeck effect and the Thomson effect. However, the total electrical power that is required to operate a thermoelectric device may be excessive when applied to the entire light engine and can limit its usefulness in lamp applications, as well as the efficiency of such applications.
Accordingly, it would be desirable to provide a light engine that resolves at least some of the problems identified above.